AnyEvent-MP/MP/Intro.pod

=head1 Message Passing for the Non-Blocked Mind

=head1 Introduction and Terminology

This is a tutorial about how to get the swing of the new L<AnyEvent::MP>
module, which allows us to transparently pass messages to our own process
and to other processes on another or the same host.

What kind of messages? Well, basically a message here means a list of Perl
strings, numbers, hashes and arrays, anything that can be expressed as a
L<JSON> text (as JSON is used by default in the protocol). Here is an example:

    [123, ["a", "b", "c"], { foo => 'bar' }]

When using L<AnyEvent::MP> you should usually use a string as first element
of a message, which describes the type of the
message. That element is called a I<tag> in L<AnyEvent::MP>, as some
API functions (C<rcv>) support matching it directly. So supposedly you want to
send a ping message with your current time to something, this is how such a
message might look like (in Perl syntax):

   ['ping', 1251381636]

The next interesting question is: Between which entities are those messages
being I<passed>? They are I<passed> between I<ports>. I<ports> are just sources
and destinations for messages. How do these ports relate to things you know?
Well, each I<port> belongs to a I<node>, and a I<node> is just the UNIX process
that runs your L<AnyEvent::MP> application.

Each I<node> is distinguished from other I<nodes>, that run on the same host or
multiple hosts in a network, by it's I<node ID>. A I<node ID> can be manually
assigned or L<AnyEvent::MP> will assign one it self for you.

Here is a visualisation about how I<nodes>, I<ports> and UNIX processes
relate to each other. The setup consists of two nodes (more are of
course possible): Node C<A> (in UNIX process 7066) with the ports C<ABC> and C<DEF>.
And the node C<B> (in UNIX process 8321) with the ports C<FOO> and C<BAR>.


  |- PID: 7066 -|                  |- PID: 8321 -|
  |             |                  |             |
  | Node ID: A  |                  | Node ID: B  |
  |             |                  |             |
  |   Port ABC =|= <----\ /-----> =|= Port FOO   |
  |             |        X         |             |
  |   Port DEF =|= <----/ \-----> =|= Port BAR   |
  |             |                  |             |
  |-------------|                  |-------------|

The strings for the I<port ids> here are just for illustrative purposes.  Even
if in reality I<ports> in L<AnyEvent::MP> are also identified by strings they
can't be choosen manually and are assigned randomly. These I<port ids> should
also not be used directly for other purposes than referring to an endpoint for
messages.

The next sections will explain the API of L<AnyEvent::MP> by going through a
few simple examples. Later some more complex idioms are introduced, which are
maybe useful to solve some real world problems.

=head1 Passing Your First Message

As start lets have a look at the messaging API. The next example is just a
demo to show the basic elements of message passing with L<AnyEvent::MP>.
It shout just print: "Ending with: 123". So here the code:

   use AnyEvent;
   use AnyEvent::MP;

   my $end_cv = AnyEvent->condvar;

   my $port = port;

   rcv $port, test => sub {
      my ($data) = @_;
      $end_cv->send ($data);
   };

   snd $port, test => 123;

   print "Ending with: " . $end_cv->recv . "\n";

It already contains most functions of the essential L<AnyEvent::MP> API.

First there is the C<port> function which will create a I<port> and will return
it's I<port id>.

That I<port id> can be used to send and receive messages. That I<port id> is a
simple string and can be safely passed to other I<nodes> in the network to
refer to that specific port (usually used for RPC, where you need to
tell the other end which I<port> to send the reply to).

Next function is C<rcv>:

   rcv $port, test => sub { ... };

It sets up a receiver callback on a specific I<port> which needs to be
specified as the first argument. The next argument, in this example C<test>, is
a I<tag> match. This means that whenever a message, with the first element
being the string C<tag>, is received the callback is called with the remaining
parts of that message.

Messages can be send with the C<snd> function, which looks like this in the
example above:

   snd $port, test => 123;

This will send the message C<['test', 123]> to the I<port> with the I<port id>
in C<$port>. The receiver got a I<tag> match on C<test> and will call the
callback with the first argument being the number C<123>.

That callback then just passes that number on to the I<condition variable>
C<$end_cv> which will then pass the value to the print. But I<condition
variables> are out of the scope of this tutorial. So please consult the
L<AnyEvent::Intro> about them.

But passing messages inside one process is boring, but before we can continue
and take the next step to interprocess message passing we first have to make
sure some things have been setup.

=head1 System Requirements and System Setup

Before we can start with real IPC we have to make sure some things work on your
system.

First we have to setup a I<shared secret>: for two L<AnyEvent::MP> I<nodes> to
be able to communicate with each other and authenticate each other it is
necessary to setup the same I<shared secret> for both of them (or use TLS
certificates).

The easiest way is to set this up is to use the F<aemp> utility:

   aemp gensecret

This creates a F<$HOME/.perl-anyevent-mp> config file and generates a random
shared secret. You can copy this file to any other system and then communicate
over the network (via TCP) with it. You can also select your own shared secret
(F<aemp setsecret>) and for increased security requirements you can even create
a TLS certificate (F<aemp gencert>), causing connections to not just be
authenticated, but also to be encrypted.

Connections will only be successful when the I<nodes> that want to connect to
each other have the same I<shared secret> (or successfully verify the TLS
certificate of the other side).

B<If something does not work as expected, and for example tcpdump shows
that the connections are closed almost immediately, you should make sure
that F<~/.perl-anyevent-mp> is the same on all hosts/user accounts that
you try to connect with each other!>

Thats all for now, there is more fiddling around with the C<aemp> utility
later.

=head1 Passing Messages Between Processes

=head2 The Receiver

Lets split the previous example up into two small programs. First the
receiver application:

   #!/opt/perl/bin/perl
   use AnyEvent;
   use AnyEvent::MP;
   use AnyEvent::MP::Global;

   initialise_node "eg_simple_receiver";

   my $port = port;

   AnyEvent::MP::Global::register $port, "eg_receivers";

   rcv $port, test => sub {
      my ($data, $reply_port) = @_;

      print "Received data: " . $data . "\n";
   };

   AnyEvent->condvar->recv;

=head3 AnyEvent::MP::Global

Now, that wasn't too bad, was it? Ok, lets step through the new functions
and modules that have been used. For starters there is now an additional
module loaded: L<AnyEvent::MP::Global>.

That module provides us with a I<global registry>, which lets us share data
among all I<nodes> in a network. Why do we need it you might ask?

The thing is, that the I<port ids> are just random strings, assigned by
L<AnyEvent::MP>.  We can't know those I<port ids> in advance, so we don't know
which I<port id> to send messages to if the message is to be passed between
I<nodes> (or UNIX processes). To find the right I<port> of another I<node> in
the network we will need to communicate that somehow to the sender.  And
exactly that is what L<AnyEvent::MP::Global> provides.

=head3 initialise_node And The Network

Now, lets have a look at the next new thing, the C<initialise_node>:

   initialise_node "eg_simple_receiver";

Before we are able to send messages to other nodes we have to initialise
ourself. The first argument, the string C<"eg_simple_receiver">, is called the
I<profile> of this node. A profile holds some information about the application
that is going to be a node in an L<AnyEvent::MP> network.

Most importantly the profile allows you to set the I<node id> that your
application will use. You can also set I<binds> in the profile, meaning that
you can define TCP ports that the application will listen on for incoming
connections from other nodes of the network.

Next you can configure I<seeds> in profile. A I<seed> is just a TCP endpoint
which tells the application where to find other nodes of it's network. To
explain this a bit more detailed we have to look at the topology of an
L<AnyEvent::MP> network. The topology is called a I<fully connected mesh>, here
an example with 4 nodes:

   N1--N2
   | \/ |
   | /\ |
   N3--N4

Now imagine another I<node> C<N5>. wants to connect itself to that network:

   N1--N2
   | \/ |    N5
   | /\ |
   N3--N4

The new node needs to know the I<binds> of all of those 4 already connected
nodes. And exactly this is what the I<seeds> are for. Now lets assume that
the new node C<N5> has as I<seed> the TCP endpoint of the node C<N2>.
It then connects to C<N2>:

   N1--N2____
   | \/ |    N5
   | /\ |
   N3--N4

C<N2> then tells C<N5> the I<binds> of the other nodes it is connected to,
and C<N5> builds up the rest of the connections:

    /--------\
   N1--N2____|
   | \/ |    N5
   | /\ |   /|
   N3--N4--- |
    \________/

Finished. C<N5> is now happily connected to the rest of the network.

=head3 Setting Up The Profiles

Ok, so much to the profile. Now lets setup the C<eg_simple_receiver> I<profile>
for later. For the receiver we just give the receiver a I<bind>:

   aemp profile eg_simple_receiver setbinds localhost:12266

And while we are at it, just setup the I<profile> for the sender in the second
part of this example too. We will call the sender I<profile>
C<eg_simple_sender>. For the sender we will just setup a I<seed> to the
receiver:

   aemp profile eg_simple_sender setseeds localhost:12266

You might wonder why we don't setup a I<bind> here. Well, there can be
exceptions to the I<fully> in the I<fully connected mesh> in L<AnyEvent::MP>.
If you don't configure a I<bind> for a node's profile it won't bind itself
somewhere. These kinds of I<nodes> will not be able to send messages to other
I<nodes> that also didn't I<bind> them self to some TCP address. For this
example, as well as some cases in the real world, we can live with this
limitation.

=head3 Registering The Receiver

Ok, where were we. We now discussed the basic purpose of L<AnyEvent::MP::Global>
and initialise_node with it's relations to profiles. We also setup our profiles
for later use and now have to continue talking about the receiver example.

Lets look at the next undiscussed line(s) of code:

   my $port = port;
   AnyEvent::MP::Global::register $port, "eg_receivers";

The C<port> function already has been discussed. It just creates a new I<port>
and gives us the I<port id>. Now to the C<register> function of
L<AnyEvent::MP::Global>: The first argument is a I<port id> that we want to add
to a I<global group>, and it's second argument is the name of that I<global
group>.

You can choose that name of such a I<global group> freely, and it's purpose is
to store a set of I<port ids>. That set is made available throughout the whole
L<AnyEvent::MP> network, so that each node can see which ports belong to that
group.

The sender will later look for the ports in that I<global group> and send
messages to them.

Last step in the example is to setup a receiver callback for those messages
like we have discussed in the first example. We again match for the I<tag>
C<test>. The difference is just that we don't end the application after
receiving the first message. We just infinitely continue to look out for new
messages.

=head2 The Sender

Ok, now lets take a look at the sender:

   #!/opt/perl/bin/perl
   use AnyEvent;
   use AnyEvent::MP;
   use AnyEvent::MP::Global;

   initialise_node "eg_simple_sender";

   my $find_timer =
      AnyEvent->timer (after => 0, interval => 1, cb => sub {
         my $ports = AnyEvent::MP::Global::find "eg_receivers"
            or return;

         snd $_, test => time
            for @$ports;
      });

   AnyEvent->condvar->recv;

It's even less code. The C<initialise_node> is known now from the receiver
above. As discussed in the section where we setup the profiles we configure
this application to use the I<profile> C<eg_simple_sender>.

Next we setup a timer that repeatedly calls this chunk of code:

   my $ports = AnyEvent::MP::Global::find "eg_receivers"
      or return;

   snd $_, test => time
      for @$ports;

The new function here is the C<find> function of L<AnyEvent::MP::Global>.  It
searches in the I<global group> named C<eg_receivers> for ports. If none are
found C<undef> is returned and we wait for the next time the timer fires.

In case the receiver application has been connected and the newly added port by
the receiver has propagated to the sender C<find> returns an array reference
that contains the I<port id> of the receiver I<port(s)>.

We then just send to every I<port> in the I<global group> a message consisting
of the I<tag> C<test> and the current time in form of a UNIX timestamp.

And thats all.

=head1 SEE ALSO

L<AnyEvent>

L<AnyEvent::Handle>

L<AnyEvent::MP>

L<AnyEvent::MP::Global>

=head1 AUTHOR

  Robin Redeker <elmex@ta-sa.org>

Revision:	1.21
Committed:	Fri Aug 28 15:58:23 2009 UTC (17 years ago) by elmex
Branch:	MAIN
Changes since 1.20:	+24 -26 lines
Log Message:	edited the intro a bit more.
#	Content
1	=head1 Message Passing for the Non-Blocked Mind
2
3	=head1 Introduction and Terminology
4
5	This is a tutorial about how to get the swing of the new L<AnyEvent::MP>
6	module, which allows us to transparently pass messages to our own process
7	and to other processes on another or the same host.
8
9	What kind of messages? Well, basically a message here means a list of Perl
10	strings, numbers, hashes and arrays, anything that can be expressed as a
11	L<JSON> text (as JSON is used by default in the protocol). Here is an example:
12
13	[123, ["a", "b", "c"], { foo => 'bar' }]
14
15	When using L<AnyEvent::MP> you should usually use a string as first element
16	of a message, which describes the type of the
17	message. That element is called a I<tag> in L<AnyEvent::MP>, as some
18	API functions (C<rcv>) support matching it directly. So supposedly you want to
19	send a ping message with your current time to something, this is how such a
20	message might look like (in Perl syntax):
21
22	['ping', 1251381636]
23
24	The next interesting question is: Between which entities are those messages
25	being I<passed>? They are I<passed> between I<ports>. I<ports> are just sources
26	and destinations for messages. How do these ports relate to things you know?
27	Well, each I<port> belongs to a I<node>, and a I<node> is just the UNIX process
28	that runs your L<AnyEvent::MP> application.
29
30	Each I<node> is distinguished from other I<nodes>, that run on the same host or
31	multiple hosts in a network, by it's I<node ID>. A I<node ID> can be manually
32	assigned or L<AnyEvent::MP> will assign one it self for you.
33
34	Here is a visualisation about how I<nodes>, I<ports> and UNIX processes
35	relate to each other. The setup consists of two nodes (more are of
36	course possible): Node C<A> (in UNIX process 7066) with the ports C<ABC> and C<DEF>.
37	And the node C<B> (in UNIX process 8321) with the ports C<FOO> and C<BAR>.
38
39
40	\|- PID: 7066 -\| \|- PID: 8321 -\|
41	\| \| \| \|
42	\| Node ID: A \| \| Node ID: B \|
43	\| \| \| \|
44	\| Port ABC =\|= <----\ /-----> =\|= Port FOO \|
45	\| \| X \| \|
46	\| Port DEF =\|= <----/ \-----> =\|= Port BAR \|
47	\| \| \| \|
48	\|-------------\| \|-------------\|
49
50	The strings for the I<port ids> here are just for illustrative purposes. Even
51	if in reality I<ports> in L<AnyEvent::MP> are also identified by strings they
52	can't be choosen manually and are assigned randomly. These I<port ids> should
53	also not be used directly for other purposes than referring to an endpoint for
54	messages.
55
56	The next sections will explain the API of L<AnyEvent::MP> by going through a
57	few simple examples. Later some more complex idioms are introduced, which are
58	maybe useful to solve some real world problems.
59
60	=head1 Passing Your First Message
61
62	As start lets have a look at the messaging API. The next example is just a
63	demo to show the basic elements of message passing with L<AnyEvent::MP>.
64	It shout just print: "Ending with: 123". So here the code:
65
66	use AnyEvent;
67	use AnyEvent::MP;
68
69	my $end_cv = AnyEvent->condvar;
70
71	my $port = port;
72
73	rcv $port, test => sub {
74	my ($data) = @_;
75	$end_cv->send ($data);
76	};
77
78	snd $port, test => 123;
79
80	print "Ending with: " . $end_cv->recv . "\n";
81
82	It already contains most functions of the essential L<AnyEvent::MP> API.
83
84	First there is the C<port> function which will create a I<port> and will return
85	it's I<port id>.
86
87	That I<port id> can be used to send and receive messages. That I<port id> is a
88	simple string and can be safely passed to other I<nodes> in the network to
89	refer to that specific port (usually used for RPC, where you need to
90	tell the other end which I<port> to send the reply to).
91
92	Next function is C<rcv>:
93
94	rcv $port, test => sub { ... };
95
96	It sets up a receiver callback on a specific I<port> which needs to be
97	specified as the first argument. The next argument, in this example C<test>, is
98	a I<tag> match. This means that whenever a message, with the first element
99	being the string C<tag>, is received the callback is called with the remaining
100	parts of that message.
101
102	Messages can be send with the C<snd> function, which looks like this in the
103	example above:
104
105	snd $port, test => 123;
106
107	This will send the message C<['test', 123]> to the I<port> with the I<port id>
108	in C<$port>. The receiver got a I<tag> match on C<test> and will call the
109	callback with the first argument being the number C<123>.
110
111	That callback then just passes that number on to the I<condition variable>
112	C<$end_cv> which will then pass the value to the print. But I<condition
113	variables> are out of the scope of this tutorial. So please consult the
114	L<AnyEvent::Intro> about them.
115
116	But passing messages inside one process is boring, but before we can continue
117	and take the next step to interprocess message passing we first have to make
118	sure some things have been setup.
119
120	=head1 System Requirements and System Setup
121
122	Before we can start with real IPC we have to make sure some things work on your
123	system.
124
125	First we have to setup a I<shared secret>: for two L<AnyEvent::MP> I<nodes> to
126	be able to communicate with each other and authenticate each other it is
127	necessary to setup the same I<shared secret> for both of them (or use TLS
128	certificates).
129
130	The easiest way is to set this up is to use the F<aemp> utility:
131
132	aemp gensecret
133
134	This creates a F<$HOME/.perl-anyevent-mp> config file and generates a random
135	shared secret. You can copy this file to any other system and then communicate
136	over the network (via TCP) with it. You can also select your own shared secret
137	(F<aemp setsecret>) and for increased security requirements you can even create
138	a TLS certificate (F<aemp gencert>), causing connections to not just be
139	authenticated, but also to be encrypted.
140
141	Connections will only be successful when the I<nodes> that want to connect to
142	each other have the same I<shared secret> (or successfully verify the TLS
143	certificate of the other side).
144
145	B<If something does not work as expected, and for example tcpdump shows
146	that the connections are closed almost immediately, you should make sure
147	that F<~/.perl-anyevent-mp> is the same on all hosts/user accounts that
148	you try to connect with each other!>
149
150	Thats all for now, there is more fiddling around with the C<aemp> utility
151	later.
152
153	=head1 Passing Messages Between Processes
154
155	=head2 The Receiver
156
157	Lets split the previous example up into two small programs. First the
158	receiver application:
159
160	#!/opt/perl/bin/perl
161	use AnyEvent;
162	use AnyEvent::MP;
163	use AnyEvent::MP::Global;
164
165	initialise_node "eg_simple_receiver";
166
167	my $port = port;
168
169	AnyEvent::MP::Global::register $port, "eg_receivers";
170
171	rcv $port, test => sub {
172	my ($data, $reply_port) = @_;
173
174	print "Received data: " . $data . "\n";
175	};
176
177	AnyEvent->condvar->recv;
178
179	=head3 AnyEvent::MP::Global
180
181	Now, that wasn't too bad, was it? Ok, lets step through the new functions
182	and modules that have been used. For starters there is now an additional
183	module loaded: L<AnyEvent::MP::Global>.
184
185	That module provides us with a I<global registry>, which lets us share data
186	among all I<nodes> in a network. Why do we need it you might ask?
187
188	The thing is, that the I<port ids> are just random strings, assigned by
189	L<AnyEvent::MP>. We can't know those I<port ids> in advance, so we don't know
190	which I<port id> to send messages to if the message is to be passed between
191	I<nodes> (or UNIX processes). To find the right I<port> of another I<node> in
192	the network we will need to communicate that somehow to the sender. And
193	exactly that is what L<AnyEvent::MP::Global> provides.
194
195	=head3 initialise_node And The Network
196
197	Now, lets have a look at the next new thing, the C<initialise_node>:
198
199	initialise_node "eg_simple_receiver";
200
201	Before we are able to send messages to other nodes we have to initialise
202	ourself. The first argument, the string C<"eg_simple_receiver">, is called the
203	I<profile> of this node. A profile holds some information about the application
204	that is going to be a node in an L<AnyEvent::MP> network.
205
206	Most importantly the profile allows you to set the I<node id> that your
207	application will use. You can also set I<binds> in the profile, meaning that
208	you can define TCP ports that the application will listen on for incoming
209	connections from other nodes of the network.
210
211	Next you can configure I<seeds> in profile. A I<seed> is just a TCP endpoint
212	which tells the application where to find other nodes of it's network. To
213	explain this a bit more detailed we have to look at the topology of an
214	L<AnyEvent::MP> network. The topology is called a I<fully connected mesh>, here
215	an example with 4 nodes:
216
217	N1--N2
218	\| \/ \|
219	\| /\ \|
220	N3--N4
221
222	Now imagine another I<node> C<N5>. wants to connect itself to that network:
223
224	N1--N2
225	\| \/ \| N5
226	\| /\ \|
227	N3--N4
228
229	The new node needs to know the I<binds> of all of those 4 already connected
230	nodes. And exactly this is what the I<seeds> are for. Now lets assume that
231	the new node C<N5> has as I<seed> the TCP endpoint of the node C<N2>.
232	It then connects to C<N2>:
233
234	N1--N2____
235	\| \/ \| N5
236	\| /\ \|
237	N3--N4
238
239	C<N2> then tells C<N5> the I<binds> of the other nodes it is connected to,
240	and C<N5> builds up the rest of the connections:
241
242	/--------\
243	N1--N2____\|
244	\| \/ \| N5
245	\| /\ \| /\|
246	N3--N4--- \|
247	\________/
248
249	Finished. C<N5> is now happily connected to the rest of the network.
250
251	=head3 Setting Up The Profiles
252
253	Ok, so much to the profile. Now lets setup the C<eg_simple_receiver> I<profile>
254	for later. For the receiver we just give the receiver a I<bind>:
255
256	aemp profile eg_simple_receiver setbinds localhost:12266
257
258	And while we are at it, just setup the I<profile> for the sender in the second
259	part of this example too. We will call the sender I<profile>
260	C<eg_simple_sender>. For the sender we will just setup a I<seed> to the
261	receiver:
262
263	aemp profile eg_simple_sender setseeds localhost:12266
264
265	You might wonder why we don't setup a I<bind> here. Well, there can be
266	exceptions to the I<fully> in the I<fully connected mesh> in L<AnyEvent::MP>.
267	If you don't configure a I<bind> for a node's profile it won't bind itself
268	somewhere. These kinds of I<nodes> will not be able to send messages to other
269	I<nodes> that also didn't I<bind> them self to some TCP address. For this
270	example, as well as some cases in the real world, we can live with this
271	limitation.
272
273	=head3 Registering The Receiver
274
275	Ok, where were we. We now discussed the basic purpose of L<AnyEvent::MP::Global>
276	and initialise_node with it's relations to profiles. We also setup our profiles
277	for later use and now have to continue talking about the receiver example.
278
279	Lets look at the next undiscussed line(s) of code:
280
281	my $port = port;
282	AnyEvent::MP::Global::register $port, "eg_receivers";
283
284	The C<port> function already has been discussed. It just creates a new I<port>
285	and gives us the I<port id>. Now to the C<register> function of
286	L<AnyEvent::MP::Global>: The first argument is a I<port id> that we want to add
287	to a I<global group>, and it's second argument is the name of that I<global
288	group>.
289
290	You can choose that name of such a I<global group> freely, and it's purpose is
291	to store a set of I<port ids>. That set is made available throughout the whole
292	L<AnyEvent::MP> network, so that each node can see which ports belong to that
293	group.
294
295	The sender will later look for the ports in that I<global group> and send
296	messages to them.
297
298	Last step in the example is to setup a receiver callback for those messages
299	like we have discussed in the first example. We again match for the I<tag>
300	C<test>. The difference is just that we don't end the application after
301	receiving the first message. We just infinitely continue to look out for new
302	messages.
303
304	=head2 The Sender
305
306	Ok, now lets take a look at the sender:
307
308	#!/opt/perl/bin/perl
309	use AnyEvent;
310	use AnyEvent::MP;
311	use AnyEvent::MP::Global;
312
313	initialise_node "eg_simple_sender";
314
315	my $find_timer =
316	AnyEvent->timer (after => 0, interval => 1, cb => sub {
317	my $ports = AnyEvent::MP::Global::find "eg_receivers"
318	or return;
319
320	snd $_, test => time
321	for @$ports;
322	});
323
324	AnyEvent->condvar->recv;
325
326	It's even less code. The C<initialise_node> is known now from the receiver
327	above. As discussed in the section where we setup the profiles we configure
328	this application to use the I<profile> C<eg_simple_sender>.
329
330	Next we setup a timer that repeatedly calls this chunk of code:
331
332	my $ports = AnyEvent::MP::Global::find "eg_receivers"
333	or return;
334
335	snd $_, test => time
336	for @$ports;
337
338	The new function here is the C<find> function of L<AnyEvent::MP::Global>. It
339	searches in the I<global group> named C<eg_receivers> for ports. If none are
340	found C<undef> is returned and we wait for the next time the timer fires.
341
342	In case the receiver application has been connected and the newly added port by
343	the receiver has propagated to the sender C<find> returns an array reference
344	that contains the I<port id> of the receiver I<port(s)>.
345
346	We then just send to every I<port> in the I<global group> a message consisting
347	of the I<tag> C<test> and the current time in form of a UNIX timestamp.
348
349	And thats all.
350
351	=head1 SEE ALSO
352
353	L<AnyEvent>
354
355	L<AnyEvent::Handle>
356
357	L<AnyEvent::MP>
358
359	L<AnyEvent::MP::Global>
360
361	=head1 AUTHOR
362
363	Robin Redeker <elmex@ta-sa.org>
364